The asinine major plot-twist that occurs half-way through the film is one of the dumbest in a teen horror, and that's saying something. The notion that two fairly normal teenage girls could be cold-blooded killers - butchers even - is so far-fetched that it could only play out this way on planet Zong, infamous for its vicious teeny-boppers: those lizard-like green girls listen to Bieber during the day then massacre innocent people at night, especially when picnicking at Lake Bodumb (their equivalent to Lake Bodom).
The percentage of psychopathy among women is notably lower than among men. (Among European men it's around 1%.) So the odds that two women of this ilk actually meet each other by accident (i.e. not in a prison, for example) is so minuscule as to be near-impossible. This is made more absurd with the fact that the blonde one suggested the murders while the brunette executed (one of) them, i.e. it wasn't just one domineering psychopath who was pushing the other one toward violence - which would have made more sense (i.e. less nonsense). What compounds this hogwash even further is their ability to hatch a plan as complex as this one and to execute it (well, at least the murdering) without a hitch - and at such short notice. Fairly idiotic fare for a movie trying pathetically to build itself on the shoulders of an infamous Finnish murder case, a REAL case. A case this movie has no actual relation to.
The least that this clueless writer could have done to alleviate some of this absurdity would have been to have the girls react with some degree of appropriate emotion right after the murders. Instead, they mostly behave as if they'd been murdering people for 100s of years! The brunette is behaving as if she'd just been to the cinema, and the blonde seems mopey and bored.
What the hell was the point of the missing car-keys? Ida dives to get them, and... she gets them. Why have that entire segment when it serves no purpose plot-wise? All that we find out is that Ida is a "great diver". Padding? Quite possibly.
The dumbest scene - aside from the "great reveal" - is the ludicrous conversation between Ida and Elias right before he gets killed. The blond kid just got butchered, in the middle of the night, in the forest, and yet Elias actually feels like striking up a conversation with Ida about some old incident involving nude photos of her. So he must be used to witnessing murder? That conversation should have taken place BEFORE the first murder. Bad Movie Writing 101. As it is, the writer is showing great incompetence in both structure and the building up of tension. When you disregard some basic rules of human psychology you inevitable sacrifice logic hence tension too. Because who's interested in a movie where people behave like aliens from planet Zong? Just give these people wings, five extra arms, and be done with it: make it a proper sci-fi film.
There is more. The girls planned this complicated double murder, the sort of intricate plan that couldn't be hatched at short notice, and yet THEY were the ones invited to the picnic - by their own victims! This is beyond far-fetched. It should be the other way round. Killers invite their victims to their own funeral, basic logic and common sense demand that. Nora says that part of the conspiracy was to "wait for an opportunity"; she could have waited for 50 years, in reality. This stinker has no logic, no sense; it's just an idiotic slasher film with asinine twists and absurd characters.
If the "whole school" saw Ida's nude photos - yet they never existed - then how can she possibly have been so convinced that they exist? Surely the words of Nora could not be enough. There would have to have been more people from the school telling her they saw the photos - or informing her that the photos don't exist. None of this adds up.
Just not to be outdone by all the moronic American thrillers who throw around plot-twists like free beer, the movie makes sure that the girls come across THE Lake Bodom killer. Or A Lake Bodom killer. It isn't clear which because the (real) killings occurred in 1960, and this particular killer doesn't appear to be anywhere near 70 or 80 years old. He is in his 40s, hence wasn't even born when the killings occur. Planet Zong logic.
Hence we add up with a typically "cute" modern-thriller finish in which different killers are pitted against each other. First the girls engage in a laughable brawl while their vehicle is still in motion. Then the Bodom killer appears, like some Jason Vorhees ghost clone, just happens to be standing on the road when the girls fight each other. The girls fly 100 meters through the air in the car, bouncing up and down the road, smashing into trees, somersaults included, yet does either get killed? Nah. Seriously injured? Not really. Because the inept writer still needs them. "The nonsense must go on", as the old thriller adage goes. As long as the movie hasn't reached the minimum feature-film length, the killer(s) must not snuff it. Otherwise what's a bad writer to do? Invent yet another anonymous killer? How is he going to get Nora to sprint like a deer if he has her injured by a spectacular bone-crushing car crash? Nora dashes out of that wreckage as if she'd just received Usain Bolt's special cocktail. No doubt, this movie should be used to test people's intelligence.
The only realistic thing here is that there is a fight that ensues between these narcissistic lesbians once Nora comes clean about her deception. Of course, even that scene had to be idiotic; they start fighting during the drive, after Nora smacks Ida right into her face with what seems to be a blunt object. Ida should have been instantly killed, or at least knocked out cold. Furthermore, the car stays on the road way too long considering that it's a) dark, b) the road is narrow and not straight, and b) the girls are at each other's throats.
Thrillers really need to start adopting and accepting the basic laws of physics, and to inject real human psychology into their characters, if they are to become great again. (Were they ever great? No. That was a little joke.) Instead, thrillers are getting exponentially dumber by the decade. Because most audiences allow them to.
The only thing that matches the writer's ineptness is the utter incompetence of the Finnish authorities in the way they handled the 1960 murders. As for the mysterious killer, there is very little doubt who he is. Read the Wikipedia page and it all becomes clear.